Microelectromechanical systems (hereinafter MEMS) require controllable, partial separation of device parts from a substrate. Compliant silicon-containing microstructures are etched so as to completely release them from an underlying silicon-containing substrate. For example, an intermediate silicon oxide layer is etched to separate at least a portion of a silicon-containing layer from a substrate.
A simple MEMS device is shown in FIG. 1. A device part, or beam 10, is partially isotropically etched from a substrate 12, leaving a support or connector 14 between them, that allows the part 10 to move, e.g., up and down, with respect to the substrate 12.
The etchant of choice heretofore for isotropically etching silicon oxide is aqueous hydrogen fluoride (HF).
A major problem with processing such parts is that as etching proceeds, adherent residues form as by-products on the substrate, and capillary, van der Walls and electrostatic attraction between the etched part 10 and the substrate 12 causes collapse of the part 10. In effect the beam 10 of FIG. 1 under this attraction bends down toward the substrate, and sticks to it, generally permanently. This phenomenon is known as stiction. In addition, etch by-products and contaminants in rinse waters also precipitate out of solution during drying steps, and cause adhesion bonding between the device part and the substrate that is even stronger than the electrostatic bonding, and interferes or prevents release of the final structure from the substrate.
Several ways of minimizing stiction have been proposed, including wet etching with HF, rinsing the residues away with a solvent, and drying the parts with a liquid that has no or little surface tension, such as supercritical carbon dioxide. An alternative etch is anhydrous HF, which does not leave residues. However, because it is a very strong acid, special equipment is required to handle it.
Since other steps in the formation of MEMS devices use dry, rather than wet methods, and large multichamber units can be used to transfer a substrate from one processing step to another without requiring that the substrate be exposed to the atmosphere, it is undesirable to mix wet and dry processes when forming such devices. The use of rinse solvents to remove moisture from a microstructure causes as many problems as it solves; the use of supercritical carbon dioxide requires a complex and difficult setup, and thus adds to the expense of manufacture.
The possibility then, of using anhydrous HF as the etchant, appears to be advantageous because it is easy to implement in a multi-chamber processor, it is an efficient, isotropic etch for silicon oxides, and it does not require mixing wet and dry processing. However, the etch rate is lower than when using aqueous HF. Further, anhydrous HF is a very powerful etchant, and can etch the materials used for making the processing chambers as well as the substrate to be etched. Thus damage to the processing chamber occurs which must be repaired, adding to the cost of manufacture.
Generally, semiconductor processes using semiconductor materials, particularly silicon and silicon oxide, are used to make MEMS devices. Because of their varying water content, doped silicon oxides, which have a high moisture content, etch faster than undoped oxides.
When an anhydrous HF etch is used to etch a silicon oxide, the amount of water present can vary depending on the water content of the silicon oxide to be etched away. Doped silicon oxides, which are hygroscopic, absorb water from the atmosphere to form internal hydroxyl groups, and thus have a high water content. Dense silicon oxides, such as thermal, undoped thermally densified TEOS and high temperatures oxides, have a lower moisture content because their water absorption is limited to the surface layer of the oxide. However, as will be further explained below, since water initiates and promotes the etch reaction between HF and silicon oxides, the presence of some water is necessary to maintain an adequate etch rate.
The overall etch reaction is4HF+SiO2 SiF4+2H2OThus water molecules are formed on the surface of the oxide during the etch step. High water content silicon oxides initiate the etch reaction rapidly and come to a steady etch rate rapidly as well. On the other hand, the etch rate of low water content silicon oxides begins slowly, i.e., there is an initiation period, and the etch rate thus increases over time. However, overall the etch rate of these oxides remains low.
Other reactions between HF and silicon oxides are also possible:6HF+SiO2→H2SiF6+2H2O  2)The silicon fluoride compound can decompose to form either silicon tetrafluoride, asH2SiF6→2HF+SiF4,  3)which does not leave a residue, and wherein the reaction products are in the gaseous phase; or to form a silicate and more HF, as2H2SiF6+3H2O→H3SiO3+6HF.  4)
This latter reaction does leave a residue which can cause stiction. Thus this reaction should be avoided to prevent deposits on the surface of the structure or feature being formed.
Further, the initial etching reaction also leaves a residue, and thus a rinse is necessary at completion of the anhydrous HF etch to remove the residue; this etch then is difficult to integrate into a multichamber or cluster tool that otherwise uses dry processes.
In efforts to solve the stiction problems, it has also been suggested to use anhydrous HF alone; but since water initiates the etch reaction, particularly for thermal oxides with a low moisture content, the etch rate for anhydrous HF alone is low. Etching with anhydrous HF can take up to 10 hours to form complex microstructures.
The addition of methanol to anhydrous HF as a substitute for water has been suggested. This would be advantageous because capillary forces are reduced, and no residue is observed on some oxides when methanol is used. However, again, the etch rate is low initially until sufficient water is generated in the reaction, which leads to a low yield; further, the unknown initiation time hinders determination of the time required for release.
The addition of acetic acid to anhydrous HF also has been suggested as a catalyst for the etch reaction with anhydrous HF, since acetic acid repels water vapor. However, the etch rate here is low as well.
Thus using anhydrous HF as the etchant results in a dry, isotropic, non-plasma etch method that does not leave a residue on the etched surface, that does not cause stiction, and that can be used in a cluster tool. However, the etch rate is too low for commercial applications.
Prior art workers have tried a combination of anhydrous HF and methanol, using an etch chamber of aluminum coated with tetrafluoroethylene. A mass flow controller regulated the flow rate of anhydrous HF and a mass spectrometer regulated the flow rate of methanol in a nitrogen carrier gas. Polysilicon cantilevers having a thickness of 2 microns, a width of 10 microns, a length of 1000 microns and a gap between the polysilicon and the substrate of 2 microns, were fabricated without stiction. The detachment length is much higher than when conventional wet release etching is performed. The etch rate however can only be estimated, at about 10-15 microns/hr at an HF partial pressure of 15 torr and a methanol partial pressure of 4.5 torr. Thus the etch rate remains low, and about 100 hours was required to etch a cantilever beam about 1000 microns long.
Thus the problem remains that by using anhydrous HF, the total time needed for release of a microstructure is long, and the etch rate cannot be known with certainty because it depends on the type of silicon oxide employed and the amount of water generated in the reaction.
An improved and more reliable method of releasing a feature from a MEM device has thus been sought that will maintain high etch rates.